File sharing' is the sharing of files over computer networks, often peer-to-peer networks. Because digital files at least theoretically don't degrade over time, with copying or in use this is a very convenient way of sharing multimedia recordings.
File sharing is a very big thing. In fact far more than half of all internet traffic is generated by file sharing. There is a big and very active scene of file sharers. Meanwhile there is even a big (political) movement for filesharing and the right of file sharing with organisations like the swedish Piratbyrån, pirate parties in Sweden, Germany, Austria, Peru, Spain, Poland, ...
BitTorrent is the biggest file-sharing system.